Method of operating a fuel cell to provide a heated and humidified oxidant

ABSTRACT

A method and apparatus are provided for humidifying fuel, and optionally oxidant, supplied to a fuel cell system, which can be a single fuel cell or a multiplicity of fuel cells. A catalytic reactor is provided, which is supplied with a portion of the fuel and the oxidant. The fuel is supplied in excess of the oxidant to the catalytic reactor, so as to generate a stream of fuel which is both heated and humidified. For a closed system, a heated and humidified fuel flow, and optionally a heated and humidified oxidant flow, are mixed with additional flows of these gases supplied to the fuel cell.

The present application is a divisional of U.S. Ser. No. 10/086,862 thatwas filed on Mar. 4, 2002, and which issued on Sep. 14, 2004 as U.S.Pat. No. 6,790,546, which is a divisional of U.S. Ser. No. 09/592,950filed Jun. 13, 2000 and which issued on Jun. 8, 2004 as U.S. Pat. No.6,746,789.

FIELD OF THE INVENTION

This invention relates to electrochemical fuels cells, more particularlyelectrochemical fuel cells which employ hydrogen as a fuel and receivean oxidant to convert the hydrogen to electricity and heat. Thisinvention is even more particularly concerned with the humidificationrequirements of such an electrochemical fuel cell employing a protonexchange membrane.

BACKGROUND OF THE INVENTION

Generally, a fuel cell is a device which converts the energy of achemical reaction into electricity. It differs from a battery in thatthe fuel cell can generate power as long as the fuel and oxidant aresupplied.

A fuel cell produces an electromotive force by bringing the fuel andoxidant into contact with two suitable electrodes and an electrolyte. Afuel, such as hydrogen gas, for example, is introduced at a firstelectrode where it reacts electrochemically in the presence of theelectrolyte and catalyst to produce electrons and cations in the firstelectrode. The electrons are circulated from the first electrode to asecond electrode through an electrical circuit connected between theelectrodes. Cations pass through the electrolyte to the secondelectrode. Simultaneously, an oxidant, typically air, oxygen enrichedair or oxygen, is introduced to the second electrode where the oxidantreacts electrochemically in presence of the electrolyte and catalyst,producing anions and consuming the electrons circulated through theelectrical circuit; the cations are consumed at the second electrode.The anions formed at the second electrode or cathode react with thecations to form a reaction product, such as water. The first electrodeor anode may alternatively be referred to as a fuel or oxidizingelectrode, and the second electrode may alternatively be referred to asan oxidant or reducing electrode. The half-cell reactions at the twoelectrodes are as follows:First Electrode: H₂→2H⁺+2e⁻Second Electrode: ½O₂+2H⁺+2e⁻→H₂OThe external electrical circuit withdraws electrical current and thusreceives electrical power from the cell. The overall fuel cell reactionproduces electrical energy which is the sum of the separate half-cellreactions written above. Water and heat are typical by-products of thereaction.

In practice, fuel cells are not operated as single units. Rather, fuelcells are connected in series, stacked one on top of the other, orplaced side by side. A series of fuel cells, referred to as fuel cellstack, is normally enclosed in a housing. The fuel and oxidant aredirected through manifolds to the electrodes, while cooling is providedeither by the reactants or by a cooling medium. Also within the stackare current collectors, cell-to-cell seals and insulation, with requiredpiping and instrumentation provided externally to the fuel cell stack.The stack, housing, and associated hardware make up the fuel cellmodule.

Fuel cells may be classified by the type of electrolyte, which is eitherliquid or solid. The present invention is primarily concerned with fuelcells using a solid electrolyte, such as a proton exchange membrane(PEM). The PEM has to be kept moist with water because the availablemembranes will not operate efficiently when dry. Consequently, themembrane requires constant humidification during the operation of thefuel cell, normally by adding water to the reactant gases, usuallyhydrogen and air.

The proton exchange membrane used in a solid polymer fuel cell acts asthe electrolyte as well as a barrier for preventing the mixing of thereactant gases. An example of a suitable membrane is a copolymericperfluorocarbon material containing basic units of a fluorinated carbonchain and sulphonic acid groups. There may be variations in themolecular configurations of this membrane. Excellent performances areobtained using these membranes if the fuel cells are operated underfully hydrated, essentially water-saturated conditions. As such, themembrane must be continuously humidified, but at the same time themembrane must not be over humidified or flooded as this degradesperformances. Furthermore, the temperature of the fuel cell stack mustbe kept above freezing in order to prevent freezing of the stack.

Cooling, humidification and pressurization requirements increase thecost and complexity of the fuel cell, reducing its commercial appeal asan alternative energy supply in many applications. Accordingly, advancesin fuel cell research are enabling fuel cells to operate withoutreactant conditioning, and under air-breathing, atmospheric conditionswhile maintaining usable power output.

The current state-of-the-art in fuel cells, although increasinglyfocusing on simplified air-breathing, atmospheric designs, has notadequately addressed operations in sub-zero temperatures, which requiresfurther complexity in the design. For instance, heat exchangers andthermal insulation are required, as are additional control protocols forstartup, shut-down, and reactant humidifiers.

Where a solid polymer proton exchange membrane (PEM) is employed, thisis generally disposed between two electrodes formed of porous,electrically conductive material. The electrodes are generallyimpregnated or coated with a hydrophobic polymer such aspolytetrafluoroethylene. A catalyst is provided at eachmembrane/electrode interface, to catalyze the desired electrochemicalreaction, with a finely divided catalyst typically being employed. Themembrane/electrode assembly is mounted between two electricallyconductive plates, each which has at least one fluid flow passage formedtherein. The fluid flow conductive fuel plates are typically formed ofgraphite. The flow passages direct the fuel and oxidant to therespective electrodes, namely the anode on the fuel side and the cathodeon the oxidant side. The electrodes are electrically connected in anelectric circuit, to provide a path for conducting electrons between theelectrodes. In a manner that is conventional, electrical switchingequipment and the like can be provided in the electric circuit as in anyconventional electric circuit. The fuel commonly used for such fuelcells is hydrogen, or hydrogen rich reformate from other fuels(“reformate” refers to a fuel derived by reforming a hydrocarbon fuelinto a gaseous fuel comprising hydrogen and other gases). The oxidant onthe cathode side can be provided from a variety of sources. For someapplications, it is desirable to provide pure oxygen, in order to make amore compact fuel cell, reduce the size of flow passages, etc. However,it is common to provide air as the oxidant, as this is readily availableand does not require any separate or bottled gas supply. Moreover, wherespace limitations are not an issue, e.g. stationary applications and thelike, it is convenient to provide air at atmospheric pressure. In suchcases, it is common to simply provide channels through the stack of fuelcells to allow for flow of air as the oxidant, thereby greatlysimplifying the overall structure of the fuel cell assembly. Rather thanhaving to provide a separate circuit for oxidant, the fuel cell stackcan be arranged simply to provide a vent, and possibly some fan or thelike to enhance air flow.

Catalytic burners are also known and operate on a principle similar tofuel cells, but at an accelerated kinetic rate and increasedtemperature. A fuel, for example hydrogen, is oxidized through directcontact with oxygen or air at a rate induced by the presence of acatalytic bed, for example, ceramic beads containing small amounts ofplatinum on the surface.

The by-product of the chemical reaction is similar to that of a fuelcell, but without any generation of electricity:O₂+2H₂→2H₂O+HEAT

The higher consumption rate of the reactants and concomitant heatrelease reflects the fact that the reaction occurs through directcontact rather than through a proton/electron transaction. Catalyticburning is flameless, and occurs at a temperature between that of a fuelcell's “cold combustion” and that of an open-flame combustion. Flow ratecan be pulsed or modulated to achieve varying heat and moistureprofiles. Hydrogen catalytic burning requires no pilot flame or spark tobe initiated.

An example of a proposal for a catalytic burner is found in an articleentitled “Catalytic Combustion of Hydrogen in a Diffusive Burner” by K.Stephen and B. Dahm at pages 1483-1492 of Catalytic Combustion ofHydrogen in a Diffusive Burner.

SUMMARY OF THE INVENTION

In the prior art, humidification through membrane stack providedhumidification only, without imparting significant temperature shift. Inthe present invention, both humidification and temperature are impartedto the stream prior to admission to the stack.

In accordance with a first aspect of the present invention, there isprovided a tubular reactor, for catalyzing reaction of hydrogen and agaseous oxidant, the tubular reactor comprising:

an elongated housing, a catalyst formed from a material adapted topromote catalytic combustion of the fuel and the oxidant, being formedinto an elongated body substantially filling the elongate housing andbeing porous, a first inlet for a gaseous fuel and a second inlet for agaseous oxidant, both first and second inlets being provided at one endof the elongated housing;

and an outlet at the other end of the housing, whereby, in use, thecatalyst promotes combustion between the fuel and the oxidant togenerate heat and moisture, whereby a heated and humidified gas flowexits through the outlet.

Preferably, the housing and the body of the catalyst are both generallycylindrical and have length substantially longer than the diameter thanthe tubular reactor.

In accordance with a second aspect of the present invention, there isprovided a fuel cell system comprising at least one fuel cell, each fuelcell comprising:

an inlet for a fuel;

an anode having a catalyst associated therewith for producing cationsfrom the fuel;

a fuel manifold, connected between the inlet and the anode, fordistributing fuel to the anode;

an oxidant inlet means for supplying oxidant;

a cathode having a catalyst associated therewith and connected to theoxidant inlet means, for producing anions from the oxidant, said anionsreacting with said cations to form water on said cathode;

an ion exchange membrane deposed between said anode and said cathode,said membrane facilitating migration of cations from said anode to saidcathode, while isolating the fuel and the oxidant from one another;

a catalytic reactor having a first inlet for fuel and a second inlet foran oxidant, and an outlet for heated and humidified gas, the catalyticreactor being mounted to supply the heated and humidified gas to thefuel cell.

Preferably, the fuel cell system comprises a plurality of fuel cells,forming a fuel cell stack.

The stack can comprise an air-breathing stack, including a plurality ofchannels extending through the fuel cell stack for permitting free flowof ambient air as the oxidant through the fuel cell stack, there beingat least one channel for each fuel cell, wherein the catalytic reactoris mounted below the fuel cell stack. The catalytic converter isconfigured to receive air as an oxidant through the second inlet thereofin excess of the stoichiometric quantity of air required for combustionof fuel within the catalytic reactor, whereby heated and humidified airis discharged from the outlet of the catalytic reactor. The outlet ofthe catalytic reactor is mounted below the channels of the fuel cellstack, whereby heated and moistened air flows upwardly through thechannels of the fuel cell stack from the catalytic reactor.

The catalytic reactor can be either generally tubular or it can bedisk-shaped, configured for flow of fuel and oxidant generally along thecentral axis of the reactor.

A further aspect of the present invention provides a method of operatinga fuel cell system comprising a plurality of fuel cells, each fuel cellcomprising an inlet for fuel, an anode having a catalyst associatedtherewith for producing cations from fuel, a fuel manifold connectedbetween the inlet and the anode for distributing fuel to the anode, anoxidant inlet means for supplying oxidant, a cathode having a catalystassociated therewith and connected to the oxidant inlet means forproducing anions from the oxidant, said anions reacting with saidcations to form water on said cathode and an ion exchange membranedisposed between the anode and the cathode, for facilitating migrationof cations from the anode to the cathode, while isolating the fuel andoxidant from one another, the method comprising

(a) supplying oxidant and fuel to the fuel cell for reaction to generateelectrical power and heat;

(b) supplying fuel to the catalytic reactor and oxidant to the catalyticreactor, in an amount greater than the stoichiometric amount requiredfor the combustion of the fuel, to ensure complete combustion of thefuel, thereby generating a flow of heated and humidified oxidant;

(c) supplying the heated and humidified oxidant to the fuel cell, forreaction with the fuel to generate electricity and heat.

For initial start-up below a preset temperature, the method can compriseinitially supplying fuel and oxidant only to the catalytic reactor togenerate a flow of heated and moistened oxidant, and passing the heatedand moistened oxidant through the fuel cell to preheat the fuel cell,and commencing supply of fuel to the fuel cell, once the fuel cellreaches a desired temperature. Then, after start-up and after the fuelcell has reached the desired temperature, a sufficient quantity of theoxidant and the fuel are supplied to the reactor, to maintain theoxidant supplied to the fuel cell system at a desired humidity level.

Yet another aspect of the present invention provides a method ofoperating a fuel cell system comprising a plurality of fuel cells, eachfuel cell comprising an inlet for fuel, an anode having a catalystassociated therewith for producing cations from fuel, a fuel manifoldconnected between the inlet and the anode for distributing fuel to theanode, an oxidant inlet means for supplying oxidant, a cathode having acatalyst associated therewith and connected to the oxidant inlet means,for producing anions from the oxidant, said anions reacting with saidcations to form water on said cathode and an ion exchange membranedisposed between the anode and the cathode, for facilitating migrationof cations from the anode to the cathode, while isolating the fuel andthe oxidant from one another, the method comprising:

(a) supplying oxidant and fuel to the fuel cells for reaction togenerate electrical power and heat;

(b) supplying fuel to the catalytic reactor and oxidant to the catalyticreactor, in an amount less than the stoichiometric amount required forcombustion of fuel, to ensure complete consumption of the oxidant,thereby generating a flow of heated and humidified fuel;

(c) supplying the heated and humidified fuel to the fuel cell, forreaction with oxidant known to generate electricity and heat.

This aspect of the method can include:

(a) providing a second catalytic reactor;

(b) supplying the second reactor with fuel and oxidant in an amountgreater than the stoichiometric amount required for combustion of fuel,thereby generating a flow of heated and humidified oxidants; supplyingthe heated and humidified oxidant to the oxidant inlet means of the fuelcell, for reaction with a heated and humidified fuel to generateelectricity and heat. In the prior art, humidification through membranestacks provided humidification only, without imparting a significanttemperature shift. In this invention, both humidity and temperature areimparted to the stream prior to admission to the stack.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

For a better understanding of the present invention and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings which show a preferredembodiment of the present invention and in which:

FIG. 1 is a schematic view of the first embodiment of a fuel cell systemin accordance with the present invention;

FIG. 2 is a schematic view of a second embodiment of a fuel cell systemin accordance with the present invention;

FIGS. 3 a, 3 b and 3 c are, respectively, perspectives, side andcross-sectional views of a tubular reactor in accordance with thepresent invention;

FIG. 4 is a plan view of part of the fuel cell stack of FIGS. 1 and 2;and

FIG. 5 is a more detailed view of the second embodiment of the fuel cellsystem shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring first to FIG. 1, the first embodiment of the apparatus isindicated generally by the reference 10 and includes an enclosure 12, inthe apparatus. In the drawings, this is identified as a HyTef-FC15Enclosure.

Within the enclosure 12, there is a fuel stack 14 comprising, in knownmanner, a plurality of PEM fuel cells, and described in greater detailin relation to FIG. 4. For the stack 14, a main fuel supply line 16 isprovided for hydrogen. The fuel line 16 extends into the enclosure 12and continues as a main supply line 18 including a respective flowcontrol valve 24 and a solenoid-operated valve 25. As shown, a stackpurge outlet at 26 enables excess hydrogen to be purged from the fuelcell stack 14. A respective solenoid controlled valve 27, controlspurging of the hydrogen gas. Again, as is known, this preventsaccumulation of contaminants and impurities in the hydrogen fuel, withinthe fuel cell stack 14. The purged hydrogen through the purge outlet 26can be recycled for consumption.

The fuel cell stack 14 in FIG. 1 is a closed stack. Hydrogen fuel flowsthrough the anode side of each individual fuel cell in known manner.Correspondingly, an air inlet 34 is provided, connected to an air line38. A pump 36 for the air is provided, and an air exhaust indicated at43.

In accordance with the present invention, the fuel or hydrogen supplyline 16 is connected to a catalytic reactor 50, which includes acatalytic reactor bed 57 (FIG. 3), comprising, for example reticulatedaluminum; this material is chosen for its thermal conductivity, cost andease of use. A separate air inlet 41 is provided, connected via a pump40 and an air supply line 42 to the catalytic reactor 50. Non-returnvalves 58 prevent back flow of air and fuel, and a flash arrestor 59 isprovided for the fuel cell.

The catalytic reactor 50 is generally tubular, has respective inlets 52and 54 for hydrogen and air, and a tubular outlet 56. A flow of heated,humidified fuel exits from the tubular outlet 56, and will then flow tothe fuel inlet of the fuel cell stack.

Reference will now be made to FIG. 2, which shows a third embodiment ofthe present invention. This embodiment of the invention again can havean enclosure, as indicated at 60, and again includes a fuel cell stack,here indicated at 62. The stack 62 here is a closed stack, and isprovided with an air pump or blower 64 connected by a main supply line66 to an inlet of the fuel cell stack 62, and excess air exhausts fromthe fuel cell stack 62 as indicated at 68.

On the hydrogen side, a hydrogen supply line 70 can include a pressuregauge and a flow meter (not shown), and comprises a main hydrogen supplyline 72 to the fuel cell stack 62 and a secondary supply line 74 to thecatalytic burner or reactor. A solenoid valve 73 is provided in the mainsupply line 72, and a solenoid valve 75, a flash arrestor 76 and anon-return valve 77 are provided in the secondary line 74. A fuel purgevalve 78 with a controlling solenoid valve 79 are provided as for thefirst embodiment.

The tubular, catalytic reactor 50 is again provided and the hydrogeninlet 52 is again provided at the side of the reactor.

An air supply line for the catalytic reactor 50 is indicated at 80 andincludes a pump or meter 82, and a respective non-return valve 84. Theair supply line 80 is connected to a respective inlet 54. Optionally, apressure gauge and a flow meter can be provided.

The outlet 56 of the tubular reactor 50 is connected by a line 85, totwo branch lines 86 and 87, which are connected by respective solenoidvalves 88 and 89 to the supply line 72 and to the air supply line 66.Although not shown, the stack 62 can optionally include a recirculationpump. Excess hydrogen can, in a known manner, be purged through theoutlet 68 or purge line 78, to prevent build-up of contaminants.

The tubular reactor 50 can be run to provide either a humidified andheated flow of air or a humidified and heated flow of hydrogen. Thesetwo modes of operation are detailed below.

To generate a flow of heated and humidified air, excess air is deliveredby the pump 82, relative to the hydrogen flow through the line 74. Inthe tubular reactor 50, the oxygen reacts with the hydrogen to generateheat and moisture. This results in a heated and moistened air flowexiting through the outlet 56. Then, the valve 88 is maintained closedand the valve 89 is opened, so that the heated and moistened air flowpasses through to the main air supply line 66, to be entrained into theair flow passing to the fuel cell stack 62.

Correspondingly, to generate a heated hydrogen flow, the valve 88 isopened and the valve 89 closed. Then, excess hydrogen is suppliedthrough the line 74, as compared to air supplied through the main fuelline 82. The flow is dead ended and is only exhausted during purgingwhen the exhaust solenoid is open. However, the flow can be controlledusing control valves when not operated in dead-ended mode. In thetubular reactor 50, the oxygen in the air reacts with some of thehydrogen to generate heat and moisture. The flow of hydrogen, withresidual nitrogen, together with heat and moisture, then exits from theoutlet 56. This flow of heated and humidified nitrogen and hydrogen gaspasses through valve 88 into the main fuel line 72.

It will be appreciated that where heated and humidified hydrogen issupplied to the fuel line 72, and as air is used as the oxidant, thisdoes result in nitrogen being injected into the fuel gas supply. Forthis reason, the purge line 78 will need to be used, to prevent thebuild-up of nitrogen within the fuel cell stack 62. Alternatively, aflowing system can be used at all times.

It is important that, in the tubular, catalytic reactor 50, completereaction takes place. In other words, it is essential that, in the twomodes of operation, residual hydrogen is not delivered to the main airline 66, nor residual oxygen delivered to the hydrogen supply line 72.This could result in potentially flammable gas mixtures of hydrogen andoxygen being delivered to the fuel cell stack 62, which is dangerous. Toensure complete reaction, proper topology and morphology of the reactormust be designed, essentially to ensure adequate residence time over thefull range of flow rates.

It will also be understood that it is possible to heat and humidify bothof the fuel and air supply lines. Because of the different requirementsof the two supply lines, this would require the provision of twoseparate tubular reactors, each of which would be configured to operatein one of the two modes outlined above.

Turning to FIG. 3, this shows, in detail, the tubular reactor 50. It isto be appreciated that this is an early version of the tubular reactor50, and in particular, the housing of the tubular reactor 50 is madefrom conventional, off-the-shelf components. It is anticipated that theoverall configuration of the tubular reactor 50 can be enhanced to givea design which both has better performance characteristics, and is moreeconomical to manufacture.

The tubular reactor 50 comprises a tubular reactor housing 51. At thelower end thereof, a T-connector 100 is provided. The T-connector 100has three coupling flanges 102, one of which is connected to the tubularhousing 51, and the two others of which provide connections for thehydrogen supply lines. At the top end, the tubular reactor 50 includes aconnector 104, again provided with connection flanges 106, one of whichis connected to the tubular housing 51 and the others of which provideconnections to supply lines. While a housing 51 of circularcross-section is shown, it will be understood that any suitablecross-section, for example a square cross-section, could be used.

Reference will now be made to FIG. 4. This shows a plan view of, forexample five pairs of flow field plates making up five individual fuelcell elements in the fuel cell stack 62. Thus, there are oxidant flowfield plates indicated at 110. Fuel flow field plates are indicated at112. Between each pair of oxidant and fuel flow field plates 110,112,there is located a respective membrane electrode assembly (MEA) and gasdiffusion media 114. Between the oxidant flow field plates 110 and theMEA 114, there are defined oxidant channels 116, and fuel flow hydrogenchannels 118 are defined between the fuel flow field plates 112 and theMEA 114. Cooling channels 120 are provided in the back of the oxidantflow field plates 110, against the fuel flow field plates 112. Thesecooling channels 120 are, like the oxidant channels 116, simply channelsextending vertically (not necessarily vertical) through the stack 62, toprovide free flow of ambient air through the channels. In known manner,other constructional details of the stack, e.g. elements holding thevarious flow field plates together, are not shown, but these can beconventional.

1. A method of operating a fuel cell system comprising a plurality offuel cells, each fuel cell comprising a fuel inlet for fuel, an anodehaving a catalyst associated therewith for producing cations from thefuel, a fuel manifold connected between the fuel inlet and the anode fordistributing the fuel to the anode, an oxidant inlet means for supplyingoxidant, a cathode having a catalyst associated therewith and connectedto the oxidant inlet means for producing anions from the oxidant, saidanions reacting with said cations to form water on said cathode and anion exchange membrane disposed between the anode and the cathode, forfacilitating migration of cations from the anode to the cathode, whileisolating the fuel and oxidant from one another, the method comprising:(a) supplying one part of the fuel to the fuel inlet of the fuel cellfor reaction at the anode to generate electrical power and heat; (b)providing a catalytic reactor for promoting reaction of the fuel and theoxidant, supplying another part of the fuel to the catalytic reactor andsupplying the oxidant to the catalytic reactor in an amount greater thanthe stoichiometic amount required for the combustion of the other partof the fuel to ensure complete consumption of the other part of thefuel, and thereby to generate a flow of heated and humidified oxidant;and (c) supplying the heated and humidified oxidant to the oxidant inletof the fuel cell system, for reaction at the cathode with the one partof the fuel to generate electricity and heat; and (d) supplying theoxidant and the other part of the fuel to the catalytic reactor, priorto supply thereof to the oxidant inlet.
 2. A method as claimed in claim1, for which it comprises, for initial start-up below a presettemperature, initially supplying fuel and oxidant only to the catalyticreactor to generate a flow of heated and humidified oxidant, and passingthe heated and humidified oxidant through the fuel cells to pre-heat thefuel cells, and commencing supply of fuel to the fuel cells, once thefuel cells reach a desired temperature.
 3. A method as claimed in claim1 or 2, which includes providing the catalytic reactor as a tubularreactor.
 4. A method as claimed in claim 1 or 2, which includes:supplying air as the oxidant providing the fuel cell system as anair-breathing system including vertical channels for flow of air as theoxidant; and providing only a portion of the air required as the oxidantthrough the catalytic reactor, with additional air flowing directlythrough the channels of the fuel cell system.
 5. A method as claimed inclaim 1 or 2, which includes supplying one part of the oxidant directlyto the fuel cells and supplying another part of the oxidant to thecatalytic reactor for heating and humidification, and mixing the onepart and the heated and humidified other part of the oxidant togetherprior to supply to the fuel cells.